In the past, retrievable packers have been designed with opposing slips above and below the packing element. This provides a means for transferring packing element loads due to differential pressure directly through the slips and into the casing without applying this load as a high-tensile load on the mandrel or other parts. This feature has proven to be very desirable for high-performance packers. One feature of such designs, however, is the tendency of the pack-off force applied to the packing element to be "bulldogged" or trapped between the two sets of opposing slip cones, which are in turn jammed under the slips. In order to release this type of a retrievable packer, the upper slip must be pulled from a position of very high engagement force between the casing and the cone which is used to guide the slip outwardly into contact with the casing. Damage can result to the tool if the slip is literally pulled off the tool, which leaves the remainder of the tool hung up in the wellbore.
In the past, various combinations of slips and cones have been used for urging the slips outwardly into contact with the casing. One such slip actuation mechanism is shown in U.S. Pat. No. 4,711,326. This patent illustrates vertically shiftable slips carried in slots by the side edges which engage mating profiles formed in the slots. These slots form guideways for the slips for shifting the slips upwardly and outwardly relative to the body between a set position for engaging a conduit and an unset position. Various other designs have used an upward pull to release the slips. Such designs are illustrated in U.S. Pat. Nos. 5,044,433 and 5,044,441.
The shortcomings of the prior designs have been in the release sequence. The prior designs have emphasized a direct pull upwardly on the uppermost slip from a mandrel to make the slips ride along the cone and retract from contact with the surrounding casing. In the prior designs, the initial contact force between the slip and the casing has been so great that shifting the mandrel to apply direct force to the slip has resulted in failure at the connection between the mandrel and the slip, leaving the slip still engaged to the casing.
The desired objective in the past has been to find a way to undermine the support for the slip so as to avoid the hazard of having to apply unduly high forces to make the slip release while under load. The apparatus and method of the present invention addresses this need to release the upper slip with a design that provides for selective weakening of the structural support for the engaged slips by virtue of components in the cone so that release can be accomplished with a greatly reduced force.